In addition to the well-known geomorphological advantages for
irrigated agriculture on the clay plain of low angle, proximity
to the White Nile, low transmission losses, and reasonably
fertile soils, some characteristics of this landscape type do
present problems for further agricultural development. The low
infiltration capacity of the clays means that much heavy rainfall
flows overland and collects in clay swales between dunes or in
depressions on the clay plain itself. As a result runoff areas
tend to have low salinity levels, whereas, conversely, run-on
areas become more saline. The salinity levels of these latter
areas are further exacerbated by the legacy of the late
Quaternary history: they were formerly swampy depressions or
maiya'a that became cut off from the White Nile as its discharge
levels decreased during the Holocene; this in turn led to salt
concentration (Williams et al., 1982). Thus surface depressions
on the clay plain could all be expected to have higher salt
concentrations than elsewhere.

Also, in the light of work by Williams (1968a) and Williams
and Adamson (1974), it seems likely that the declining levels of
the White Nile during the early Holocene led to concentrations of
salts in the "littoral" zones. This could pose further
problems for development, not only on the clay plain but also
much farther to the west. Groundwater seepage from the White Nile
passing westwards through such areas would seem unwise since the
nature of the groundwater would aggravate the salinity problem.
It must be stressed that salinity is not only a function of
present day rainfall distribution but is ultimately bound up with
the changing history of the White Nile. The dominant controls on
salinity are soil texture, permeability and topography, all of
which are inextricably linked to the depositional history of the
river. Another related legacy of the late Quaternary history is
the presence locally of underlying permeable beds of silts and
sands. Unlined irrigation canals cutting across such areas could
suffer considerable water losses through seepage.

The Qoz

Salinity poses problems here on the run-on areas of clay
swales between dunes. The sands themselves naturally have high
infiltration capacities and by comparison with other sediment
types have poor water-retentive properties. There are, however,
small but important differences between the pale and the red
sands: the latter, owing to their haematitic coating, are
slightly more water-retentive (Vail, 1982).

There is a considerable problem to agriculture on the qoz
areas from sand movement. This is not so much a problem of dune
encroachment, for their margins at least appear to be fixed, and
only a few small, active barchan dunes of fine sand were observed
in the area. Rather it is a problem of windblown sand moving
across areas with perhaps a quite thin sand cover and
accumulating around individual plants so that crops tend to
become choked; at best their growth is impaired.

In such an arid marginal region it is clear the sands offer
little potential, at least for annual crops. Any injudicious use
of such lands for cultivation or heavy grazing would have serious
repercussions, increasing the existing problems created by even
small quantities of moving sand and by creating conditions
conducive to the development of more active dunes to threaten
irrigation schemes on the clay plains.

Hills and Pediments

Of the three landscape elements, these areas would seem to
offer the least attraction for agricultural development. The
underlying Basement Complex, which constitutes most of the rocks
of this landscape type, yields comparatively thin, poor regoliths
and groundwater supplies are likely to be confined to khor beds.
The Nubian Sandstone Series, which underlies the pediments in the
north, and the Umm Ruwaba Series of the central and southern
parts of the research area both provide greater potential for
groundwater development, though the characteristics of these
deposits in some parts may render the water too saline for human
or even animal consumption.

Future Research

Clearly further work on the late Quaternary history of the
area would be not only of academic interest but also valuable in
assessing development potential. Excellent detailed work by M. A.
J. Williams, D. A. Adamson, and associates on the nature of the
sediments of the Gezira Plain and on the bank of the White Nile
has made possible the reconstruction of the late Quaternary
history of the area and demonstrated some important implications
to future agricultural development, specifically with regard to
salinity levels. A similar approach in the research area would
have comparable results.

Detailed examination of the hydrology of the qoz area to the
west would also have similar benefits in assessing development
potential.

With further work in this area it would be possible to produce
a useful geomorphologically based land-use capability map. A
tentative one, based largely on soil character, is attempted for
the research area north of 14 30'N, and extending further
northwards as far as 15 371/2'N, in the next chapter. It is
believed that the geomorphological characteristics discussed in
this chapter are also applicable to this area to the north and
that the comments upon soils in the next chapter relating to the
more northerly area are also valid for the area discussed in the
present chapter.

Acknowledgements

The financial support of the Inter-University Council (now
Higher Education Division of the British Council) is gratefully
acknowledged. Support from my own colleagues at the University
College of Swansea and from the Department of Geography of the
University of Khartoum was invaluable. The assistance of the
Sudan Survey Department in agreeing to the release of the aerial
photographs is acknowledged. I should like to thank Dr. D. S.
Brown, who kindly identified the molluscs. Finally, I thank all
the people I met in the research area for their unfailing
generosity and hospitality.

Adamson, D. A., M. A. J. Williams, and R. Gillaspis.1982.
"Palaeogeography of the Gezira and of the Lower Blue and
White Nile Valleys." In M. A. J. Williams and D. A. Adamson,
eds., A Land between Two Niles, pp. 165-219.

---. 1976. "The Origins of the Soils between the Blue and
White Nile Rivers, Central Sudan, with Some Agricultural and
Climatological Implications." Economic and Social Research
Council Occasional Paper 6. Khartoum.

---. 1980. "Late Quaternary Depositional History of the
Blue and White Nile Rivers in Central Sudan." In M. A. J.
Williams and H. Faure, The Sahara and the Nile, pp. 281-304.

---. eds.1982. A Land between Two Niles. Balkema, Rotterdam.

Williams, M. A. J., D. A. Adamson, and H. H. Abdulla. 1982.
"Landforms and Soils of the Gezira: A Quaternary Legacy of
the Blue and White Nile Rivers." In M. A. J. Williams and D.
A. Adamson, A Land between Two Nilos, pp. 111-142.

The aims of pedological study in the Third World have been
outlined by Moss (1968). These are, first, to understand the
distribution and mode of formation of soils so that existing land
use may be improved or new techniques introduced, and, second, to
promote a greater knowledge of the land, and particularly of the
soils, to enable predictions to be made about the behaviour of
the land under proposed land-use changes. The prime aim of this
study is to identify those characteristics of the soils of
Western White Nile which would encourage or inhibit the
introduction of rural change, bearing in mind the narrow
environmental constraints within which the cultivator has to
operate in such arid marginal lands.

The study area extended northwards from 14º30' to 15º371/2'N
and from the White Nile westwards as far as far as 32 ºE (fig.
2.2). Although only part of the project area lies within this
area of study the author had the opportunity of touring the
project area to the south and believes that his findings are
applicable to the whole of that area. The findings are based on
visual examination and preliminary analysis in the field, aerial
photography and satellite imagery, and laboratory analysis of
field samples collected in September and October 1980.

In Sudan, pedological knowledge is very scanty, although the
country's economy depends totally on crop and livestock
resources. The vastness of the country has always been blamed as
the cause of a lack of detailed research in many fields of study.
In the present field, the only area which has been studied in
relative detail is the Gezira, which accounts for not more than
1-2 per cent of the entire area of the country.

Compared with the Central Clay Plain of the Blue Nile, the
alluvial plains of the White Nile, particularly on the west bank,
have had little agricultural development until recently, and so
very few semi-detailed soil surveys have been carried out; almost
all of those that have have been confined to small agricultural
schemes. Detailed information on these surveys is given by Ali
(1970), Salih (1975), Younis (1977), and El Fadl and El Sayed
(n.d.). In practice not all the soil survey reports are
available.

The study area lies within the Sahel zone, which stretches
from the Atlantic to the Indian Ocean between 11 and 15 N.
According to Mensching and Ibrahim (1977) this zone is considered
to be most vulnerable to the processes of desertification. At a
conference held at Khartoum in September 1980, it was announced
that some 700,000 square kilometres of semi-arid Sudan are
threatened by desertification. Whether this figure is exaggerated
or not, it still gives concern for the future of Sudan's soil
resources. The apparently continuing decline in annual rainfall
totals since 1969, in an area with a long dry season of eight to
ten months, exposes soils to aeolian erosion, with the result
that enormous quantities of fertile topsoil are blown away from
the Sahel zone every year.

Increased cultivation and overgrazing of the desert margins
encourages wind erosion by creating larger areas of minimal
vegetation cover. Research in Northern Darfur Province in 1979
has confirmed that millet cultivation has extended desertwards
beyond the normal climatic limits and has been the major factor
in the process of desertification. Degradation of soil and a
decrease in agricultural productivity have gone hand in hand.
Though there is evidence to show that similar processes are
occurring in the White Nile area, the results obtained in
Northern Darfur must be considered of only limited applicability,
as soil conditions differ appreciably between the two areas.

Physical Characteristics of the Study Area

Soil characteristics depend very largely upon the parent
materials (see also chapter 1), the climate (ch. 3), the
availability and movement of soil water, and the vegetation which
develops (ch. 4).

The rocks of the study area (fig. 1.1) mainly belong to the
Nubian Sandstone Series, which overlies the Pre-Cambrian Basement
Complex. The former comprises quartzites, yellowish brown
sandstones, mudstones, and basal conglomerates. Its mineralogy is
mostly quartz with feldspars and a variety of ferruginous and
manganic minerals. This series provides one of the most
significant water-bearing aquifers in the Sudan wherever it
exists in substantial thickness, and contributes significantly in
this area as the parent material to its soils. The Basement
Complex comprises an assemblage of igneous and metamorphic rocks
outcropping in only a very few localized areas. Basement Complex
outcrops are believed by some to be the "parent
material" of the sediments that later formed the Nubian
Sandstone Formation (Hussein, 1976). Superficial deposits of qoz
sands and river and wadi alluvia are the most recent geological
units in the area. Stabilized sand dunes such as Qoz Abu Dulu
cover the entire western part, extending beyond longitude 32° E.

Six physiographical units of pedological significance may be
identified:
- White Nile alluvium of the flood plain.
- Wadi alluvial plains.
- Sand dune areas (the qoz).
- The inland plains (with "red soils").
- Partially weathered outcrops
- Hilly areas and gravel mounds of the Nubian Sandstone. A
schematic cross-section from west to east explains the
association of these geomorphic units and their contribution to
soil development (fig. 2.1). Immature soils prevail on steeper
slopes, whereas the lowlands have experienced more advanced
development in soil profiles. (Further details of geomorphology
are considered in chapter 1.)

The study area's regional position means that the climate is
hot and dry for most of the year (see figs. 3.1 and 3.2). The
number of months in which precipitation exceeds (potential)
evapotranspiration is less than one throughout the whole area
(Davies, 1973). Such a climatic regime suggests a transition
southwards from desert detritus in the far north through
varieties of arid and semi-arid soils with improved maturity of
development towards the south. (Climate is discussed in chapter
3.)

Apart from the waters of the White Nile, two other resources
exist: surface waters confined mainly to wadi flood plains, and
groundwater from the Nubian Sandstone aquifers. The Nubian
Sandstone aquifers provide an alter native source to surface
water. Today, full use is made of groundwater at the El Rawakeeb
and Gadain mixed farming schemes.

The pilot scheme at El Rawakeeb, which started in the early
1970s, continues to be successful. Besides helping the nomads
with their livestock, this scheme enables cheap poultry to be
reared for sale in the Three Towns. At first water was found to
be moderately saline and very hard, but continual pumping has
proved the salinity to be only superficial. With careful pumping
and proper selection of freshwater aquifers this water resource
in the Nubian Series could prove most useful in developing
agriculturally areas to the west of the White Nile. Such
possibilities, together with the use of Nile waters for
irrigation, make a proper assessment of these soils imperative.

The different geomorphic units are characterized by different
combinations of grass and tree species; that is, the units are
ecological as well as pedological, though the former have less
sharp boundaries than the latter. Field observations show clear
signs of degradation of the vegetation. Worrall (1960) concluded
that vegetation in the semi arid and arid zones of the Sudan is
determined mainly by soil moisture conditions. Kassas (1956)
demonstrated clearly, in the desert and semi-desert areas to the
west of the Nile, the detrimental effect of man's cultivation and
the grazing of his animals upon the natural vegetation,
especially the grass cover, and hence the detrimental effect upon
the soil.

Soil Characteristics General Description of the Soils

The geomorphic units identified from aerial photography and
satellite imagery have been considered as basic soil units. Some
workers regard them as comparable to soil series and as such they
have been widely used in many parts of Africa (Areola, 1977;
Moss, 1968: Tomlinson, 1970).

The soils west of the White Nile have developed on flood
plains and recent terraces of former courses of the White Nile
(Williams and Adamson, 1974). Other soils further west include
those formed on stabilized sand dunes and on wadi alluvia. Soils
derived from Nubian Sandstone parent material are quite extensive
and fall into two units: the "red soils" and the
"immature lithomorphic soils." The delineated units are
described below.

Soils developed on this landform are of very fine texture with
clay-sized material forming the dominant particle size. The
matrix is generally non-calcareous, although dark-grey inert
calcium carbonate concretions are often encountered throughout
the profile. Field investigation reveals that the soils are dark
grey to dark brownish-black, slowly permeable, cracking clays.
Their high shrink-swell properties suggest a dominant
montmorillonite clay mineral. These soils are comparable with the
Central Clay Plain soils of the Blue Nile region. Minor
differences in physical, mineralogical, and chemical properties
exist. No attempt was made to differentiate between the recent
and older terraces of the White Nile flood plain in this study.
The soils generally belong to the vertisol group (FAO-Unesco, 1
974 ).

Wadi Alluvial Plains

These are soils formed along the wadi flood plains west of the
White Nile river. They frequently show many features associated
with vertisols, such as shrinking and swelling, formation of deep
cracks, slickensides and gilgai features. These phenomena could
be attributed to the presence of montmorillonitic clay minerals,
but their clay percentages are insufficient for them to qualify
as vertisols. They are somewhat stony, but otherwise fertile,
being extensively utilized for dry cultivation of sorghum and
vegetables in particular These dark brown, silty clay loams could
qualify as fluvisols. Wadi Afu, Wadi El Hal and Waarat El Hamdab
are examples from the southern part, while from the northern part
Khor Barok and Wadi Abu Heleifi are considered classic examples.
Substantial variations in texture, nutrient content, and colour
exist between these soils in the various parts of the study area.

Sand-dune Areas (Qoz)

These are mainly associated with stabilized sand dunes but
where cultivated are frequently exposed to wind erosion. They
occur mainly in the western part of the study area and more
precisely along the giant dune of Qoz Abu Dulu. Although the
variation in climate is not abrupt, noticeable differences in
soil and vegetation exist when the northern and southern parts
are compared. The soils on this zone are fully utilized for
millet and groundnut cultivation despite the scarcity of
precipitation (<200 mm per annum). This is because plants are
able to take full advantage of the wet season moisture
accumulated at depth by the high permeability of these sandy
soils. Scarcity of groundwater for domestic animal and crop use
makes this zone vulnerable to crop failure during drought
periods. Termites have some influence on soil development in the
area, most probably in a retrogressive manner. This is because
they play a part in completely destroying the vegetation and
hence inducing desertification. These soils are classified as
regosols.

These "red soils" are the dominant soils north of 14
45'N. They are mostly sandy or silty clay loams similar to the
wadi alluvia in clay content but quite distinct as far as colour
and other physical and chemical properties are concerned. They
probably form a complete catena with the immature and hilly soils
mentioned below. The factor of topography plays an important role
in the formation of these soils. The agents of erosion, wind and
precipitation, have also influenced their development. The
underlying strata is usually the Nubian.

General field observations show that the landscape is flat
with minor undulations covered with a thin film of mobile sand. A
cracking crust, often surficial, frequently occurs where wind
erosion is intense and the sand removed. The soils are easily
permeable by water, but lower in the profile moderately hard
layers of ironstones and gravelly sandstones occur at variable
depths. They have been classified as yermosols.

Partially Weathered Outcrops, and Hilly Areas and Gravel
Mounds of the Nubian Series

These are extensive in the western part of the northern
section, but are of little agricultural consequence. They may be
classified as lithosols.

Methods of Investigation

In the field at least one profile was selected to represent
each of the delineated units and these 25 profiles were described
according to the current procedures used in the Sudan Soil Survey
Administration (El Fadl and El Fadl, 1973; Nykango, 1973). Three
samples were taken from each profile, air dried and packed in
polythene bags. They were analysed for their chemical and
physical properties in the pedology laboratory of the Department
of Geography, University College of Swansea. The location of each
profile is indicated in figures 2.2 and 2.3.

The Profiles

Six soil profiles have been selected for description as
representative of the soil units delineated.

White Nile Flood PlainSite 1: VertisolLocation: El Alaqa riverine alluvial plain.Parent material: White Nile alluvium.Relief: Flat.Natural vegetation: Nile reeds and a few acacia trees.Climate: Semi-arid.Land use: None at time of visit due to high reservoir
level; during the dry season gerf (river-bank) cultivation is
maintained. Sorghum and vegetables are grown. Cotton can do well
here.

30-70 cm.10 yr 2/1 (moist), dark brownish-black, maintains
some physical characteristics as above except for the decrease in
size of cracks; very few rootless, nonpermeable; slickensides are
apparent; rather massive and much less permeable than above.

Location: Umm Sidr-northern part of Qoz Abu Dulu.Topography: Undulating; top of fixed sand dune, sloping
east and west, but site area generally flat.Natural vegetation: A few grasses dominantly (haskanit,
30 cm tall) and a few scattered small acacia trees.Parent material: Sand deposited by wind action, some
locally but mostly from the north.Climate: Semi-arid, but much drier than the rest of the
area under study.Land use: Millet cultivation is the dominant activity in
the surroundings, with rough grazing in spite of the dry climate.

Profile description:
0-10 cm. 5 yr 6/6 (dry), orange, friable sand with no
horizonation; lamination due to wind deposition is apparent; no
organic matter, no rootless; some dark mineral grains; at the
base a concentration of coarser sands, indicating basal
deposition on top of a moist surface, is encountered; faint but
well-defined boundary; very permeable and very porous.

Soil pH was determined by shaking a soil-water mixture (soil
to water ratio 1:5) for two hours and taking pH readings with a
digital pH-metre. Particle size analyses were performed by the
pipette method. Loss on ignition, moisture content, electrical
conductivity, organic carbon and organic matter, calcium
carbonate and the rest of the chemical analyses were performed
following the procedures used by the Sudan Soil Survey
Administration, Wad Medani. The results obtained were employed to
classify the soils as well as to determine tentatively land
capability and potential productivity.

These properties are relevant to soil use and have a practical
application in the field as well as for classification. Soil
orders identified in this study are according to the FAO-Unesco
system of 1974 and imply the various processes dominant in their
genesis. Soil classification was limited to orders and subgroups
only, as a reconnaissance-level survey does not warrant more
detailed classification.